The aim of this study is to evaluate the prevalence and prognosis of febrile seizures, which is the most common convulsive disorder in any age group. Because it has an established base population, the NINCDS Collaborative Perinatal Project has the denominator data for a quantitative statement of risks. Because these were prospectively documented early examinations, this study could include information on neurological status of children before any seizure. An initial report dealt with the likelihood of development of chronic epilepsy in children who have experienced febrile seizures. A subsequent study indicates that intellectual deficit and early learning disorder are not more frequent in children with febrile seizures only. Work now completed has delineated the remaining risks with febrile seizures. The quantification of these risks is an important element in development of a rational approach to clinical management.